feat(deploy): gencode 纳入 server 发版产物 + EC2 生码封装

server 流水线编译并发布 jiu-gencode 二进制（回滚路径随 release 资产
自动下载），部署到 /opt/jiu/backend/，对旧版回滚做存在性判断优雅跳过。
新增 deploy/jiu-gencode.sh 封装自动读 production.env，线上生码从此
一条 ssh 即可，无需本地交叉编译 + scp。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Wdwo7SmgBJU37cBrkhPK

scripts/ci/release-server.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # release-server.sh <tag>  —  create the Forgejo Release for the backend pipeline
-# and upload jiu-server + configs.tar.gz.
+# and upload jiu-server + jiu-gencode + configs.tar.gz.
 set -euo pipefail
 
 # shellcheck source=scripts/ci/lib-forgejo.sh
@@ -20,6 +20,7 @@ PYEOF
 
 create_release "$TAG" "$BODY"
 upload_asset dist/jiu-server
+upload_asset dist/jiu-gencode
 upload_asset dist/configs.tar.gz
 
 echo "==> release-server: done — Release ${TAG} created"